amplitude,
and speed. Wavelength is the measureddistance
(in degrees longitude) between successivewaves.
The measurement is usually taken fromtrough
to trough, ridge to ridge, or from any pointon
one wave to the same corresponding point onthe
next wave. The amplitude is one-half of thewave’s
total range, which is measured in degreeslatitude
from the peak of the ridge to the base ofthe
trough. Figure 8-3-1 illustrates the measure-mentof
wavelength and amplitude on a longwave.
Also, note the short wave on the long wave.The
speed of waves is usually governed by theirlength.
The longer the waves, the slower theymove,
and vice versa.

Long Waves

A significant feature of the westerlies in both

hemispheres,
long waves vary in length from 50°to
120° longitude, have large amplitudes, and are

Figure 8-3-1.—Illustration of a long and short wave and the

measurement
of length and amplitude of a longwave.

slow moving. In the overall hemispheric pattern there
are normally four or five long waves inexistence
at any one time. However, there aretimes
when there are as many as seven or as fewas
three. The pattern is a persistent feature, andwaves
do not appear or disappear rapidly. Achange
in the number of waves in the pattern issignificant.
The fewer the number, the moreprogressive
are the weather patterns at the surface.The
greater the number, the more stagnant theweather
patterns. It is during these periods thatprolonged
good or bad weather affects a region.The
number of waves and pattern changes areoften
discussed at briefings for meteorologists.New
long waves form from short waves or achanging
synoptic situation, and their develop-mentis
associated with the development of newintense
circulations at lower levels.

Because some short waves have large ampli-tudes, it
is often difficult to distinguish them fromlong
waves. Also, it is virtually impossible toidentify
wave types (long or short) on a singlechart.
A series of charts over a 3- to 5-day periodis
best for this. This is usually enough time formost
short waves to move through the slowerlong-wave
pattern, thereby distinguishing betweenthe
two types. The long waves have a normalmovement
at 40°N of about 2° longitude per dayin
the spring to less than 1° per day during thefall.
They can also become stationary or evenretrogress.

Because a long wave’s amplitude increases with
height within the troposphere (greater at 300mb
than at lower levels) the long-wave pattern isbest
identified at the 300-mb level. Here, the wavecontours
are approaching their maximum ampli-tude,and
the overall pattern is smooth (no shortwave
distortion). The increase in amplitude withheight
also distinguishes long waves from shortwaves.
Short waves often disappear with heightand
may not be detectable above 500 mb. Thisis
attributable to the temperature patternsassociated
with the two types of waves. With longwaves,
the troughs are cold and the ridges warm;the
opposite holds true for short waves.

Synoptically, long waves are related to a number
of weather occurrences. Figure 8-3-2illustrates
many of the relationships. Simplystated,
the weather between the long-wave troughand
the downstream ridge (trough sector) is moreapt
to be bad than the weather between the troughand
the upstream ridge (ridge sector).

Figure 8-3-2.—Long waves and related weather.

Short Waves

Superimposed on the long-wave contours of a
given upper-air chart, say 500-mb, are numerousshort
waves. Ten or more short waves are presentin
the hemisphere most of the time. They haveshorter
wavelengths and smaller amplitudes andmove
faster than long waves. They move in thesame
direction as the current in which they areembedded.
Their eastward motion is very nearthat
of the 700-mb flow. Their normal movementis
on the order of 8° longitude per day in summerand
12° per day in winter. Short waves areprogressive
and never retrograde. Their troughsare
warm and their ridges cold; therefore, theydo
not extend to great heights and are mostpredominant
in the lower half of the troposphere(500
mb and below).

Short waves have a great effect on long waves. They
dampen (flatten) long-wave ridges as theymove
across them, and at the same time, the shortwave
is weakened. As a short wave approachesa
long-wave trough, the short wave strengthensand
the long-wave trough intensifies. This latteroccurrence
often results in the formation of asurface
low-pressure system (cyclogenesis).The
location of short waves coincides with thesmall
closed height fall centers (troughs) andheight
rise centers (ridges) of a 700- or 500-mbtime-differential
chart.